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Hiotographic 

Sciences 

Corporation 


33  WEST  MAIN  STMET 

WnSTU.N.Y.  I4S«C 

(716)  •73-4S03 


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CIHM/ICMH 

Microfiche 

Series. 


CIHM/ICMH 
Collection  de 
microfiches. 


Canadian  Institute  for  Historical  Microreproductions  /  Institut  Canadian  da  microraproductions  historiques 


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TAchnicai  and  Bibliographic  Notes/Notes  tecliniques  at  bibliographiquas 


Tha  inatituta  has  attamptad  to  obtain  tha  bast 
original  copy  availabia  for  filming.  Faaturas  of  this 
copy  which  may  be  bibliographically  uniqua, 
which  may  alter  any  of  tha  imagas  in  tha 
raproduction,  or  which  may  significantly  changa 
tha  usual  mathod  of  filming,  ara  chackad  balow. 


n 


Colourad  covars/ 
Couvartura  da  couleur 

Covars  damagad/ 
Couvartura  endommag6a 

Covars  rascorad  and<'or  laminatad/ 
Couvartura  rastaurte  at/ou  nalllcul6a 


I      I   Cover  title  missing/ 


La  titra  da  couverture  manque 


I      I    Coloured  maps/ 


Cartas  g^ographiquas  en  couleur 


□    Coloured  ink  (i.e.  other  than  blue  or  black)/ 
Encra  da  couleur  (i.e.  autre  que  bleue  ou  noire) 

I      I    Coloured  plates  and/or  illustrations/ 


E 
D 


D 


n 


Planches  et/ou  illustrations  en  couleur 


Bound  with  other  material/ 
RelW  avac  d'autres  documents 


Tight  binding  may  causa  shadows  or  distortion 
along  interior  margin/ 

La  re  liure  serrie  peut  causer  de  I'ombre  ou  de  la 
distortion  la  long  de  la  marge  int^rieure 

Blank  leaves  added  during  restoration  may 
appear  within  the  text.  Whenever  possible,  these 
have  been  emitted  from  filming/ 
11  se  peut  que  cartainas  pages  blanches  aJoutAes 
lors  d'une  restauratlon  apparaissent  dans  la  texta, 
mais,  iorsque  cela  Atalt  possible,  ces  pages  n'ont 
pas  AtA  fiimAes. 

Additional  comments:/ 
Commentairas  supplAmentaires: 


L'Institut  a  microfilm^  la  mellleur  exemplaire 
qu'll  lui  a  4t6  possible  de  se  procurer.  Les  details 
do  cat  exemplaire  qui  sont  peut-Atre  uniques  du 
point  de  vue  bibliographiqua,  qui  peuvent  modifier 
une  image  reproduite,  ou  qui  peuvent  exiger  une 
modification  dana  la  mithoda  normala  da  filmaga 
sont  indiqute  ci-iS^ssous. 


r~l    Coloured  pages/ 


D 


This  item  Is  filmed  at  tha  reduction  ratio  chackad  balow/ 

Ca  document  est  filmi  au  taux  da  reduction  indiqui  ci-daasous. 


Pagea  de  couleur 

Pages  damaged/ 
Pages  endommagtes 

Pages  restored  and/oi 

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I — I    Pages  damaged/ 

I — I    Pages  restored  and/or  laminated/ 


0    Pages  discoloured,  stained  or  foxed/ 
Piges  dteoiorAas,  tachattes  ou  piqutes 

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Pages  ditachtes 

HShowthrough/ 
Transparence 

□    Quality  of  print  varies/ 
Quality  InAgale  da  I'lmpreasion 

□    includes  supplementary  material/ 
Comprend  du  material  suppl6mentaire 

□    Only  edition  available/ 
Saule  Mition  disponible 


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Pages  wholly  or  partially  obscured  by  errata 
slips,  tissues,  etc.,  nave  been  refilmed  to 
ensure  the  beat  possible  image/ 
Les  pages  totalemant  ou  partieilemant 
obscurcias  par  un  fauiilat  d'arrata,  una  pelure, 
etc.,  ont  4ti  filmtes  k  nouvaau  da  fa^on  h 
obtanir  la  maiiiaure  image  possibla. 


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The  copy  filmed  here  has  been  reproduced  thanks 
to  the  generosity  of : 

University  of  Saskatchewan 
Saskatoon 

The  images  appearing  here  are  the  best  tjuality 
possible  constidering  the  condition  and  legibility 
of  the  original  copy  and  in  keeping  with  the 
filming  contract  specifications. 


Original  copies  in  printed  papnr  covers  are  filmed 
beginning  v/ith  the  front  covef  and  ending  on 
the  last  paf;e  with  a  printed  or  illustrated  impres- 
sion, or  the  back  cover  when  appropriate.  All 
other  original  copies  are  filmed  beginning  on  the 
first  page  with  a  printed  or  illustrated  impres- 
sion, and  ending  on  the  last  page  with  a  printea 
or  illustrated  impression. 


The  last  recorded  frame  on  each  microfiche 
shall  contain  the  symbol  ^^>  (meaning  "CON- 
TINUED"), or  the  symbol  ^  (meaning  "END"), 
whichever  applies. 

IVIaps.  plates,  charts,  etc.,  may  be  filmed  at 
different  reduction  ratios.  Those  too  large  to  be 
entirely  included  in  one  exposure  are  filmed 
beginning  in  the  upper  left  hand  corner,  left  to 
right  and  top  to  bottom,  as  many  frames  as 
required.  The  following  diagrams  illustrate  the 
method: 


L'exemplaire  f ilm*  fut  reprodult  grice  A  la 
gin^rosit^  de: 

University  of  Saskatchewan 
Saskatoon 

Les  images  suivantes  ont  tti  reproduites  avec  ie 
plus  grand  soin,  compte  tenu  de  la  condition  et 
de  la  netteti  de  rexemplaire  filmA,  et  en 
conformity  avec  les  conditions  du  contrat  de 
filmage. 

Les  exemplaires  originaux  dont  la  couverture  en 
papier  est  imprimte  sont  film6s  en  commenpant 
par  ie  premier  plat  et  en  terminant  soit  par  la 
dernlAre  pege  qui  comporte  une  empreinte 
d'impression  ou  d'illustratlon,  soit  par  Ie  second 
plat,  salon  Ie  cas.  Tous  les  autres  exempilaires 
originaux  sont  filmfo  en  ccmmenpant  par  la 
premiere  page  qui  comporte  une  empreinte 
d'impression  ou  d'illustratlon  et  en  terminant  par 
la  dernlAre  page  qui  comporte  *in9  telle 
empreinte. 

Un  des  symboies  suivants  apparattra  sur  la 
dsrnlAre  image  de  cheque  microfiche,  selon  ie 
cas:  Ie  symbols  -^  signifie  "A  SUIVRE",  Ie 
symbole  ▼  signifie  "FIN". 

Les  cartes,  planches,  tableaux,  etc.,  peuvent  Atre 
filmte  A  des  taux  de  rMuction  diffirents. 
Lorsquj  Ie  document  est  trop  grand  pour  Atre 
reproduit  en  un  seul  clichA,  il  est  film*  A  partir 
de  Tangle  supArieur  gauche,  de  gauche  i  droite, 
et  de  haut  en  has,  en  prenant  Ie  nombre 
dMmages  nAcessaire.  Les  diagrammes  suivants 
illustrent  la  m*thode. 


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OF   "Hie 


A   DISCUSSION 


OP   THB 


GENERAL  PRINCIPLES  INVOLVED 


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CONSTRUCTION  AND  ACTION 


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Isometrical  Truss  Bridge. 


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BY 


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CHARLES  MACDONALD,  C.E. 


I 


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PHILADELPHIA: 

'r.   p:  L  L  w  o  o  D  z  e  l  l. 

18()7. 


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TO  THE  PUBLIC. 


•*<'^ii 


I'    f'.V*' 

^  '(•'.••■■jI 


An  c'xporionce  of  nearly  a  (luartor  of  a  century  in 
construct  in  jj;  and  reconstructing  railway  bridges  under 
the  most  diifi''ult  circumstances,  and  the  observations 
of  a  professional  career  extending  over  a  much  longer 
period,  having  convinced  the  subscriber,  that  there  are 
more  errors  in  bridge  architecture  irom  the  employ- 
ment of  elements  which  are  superfluous  and  deranging, 
tliun  from  the  lack  of  those  which  are  sustaining  and 
ivciprocating.  a  sericH  of  experiments  was  instituted, 
extending  througli  several  years,  under  circumstances 
t'livoralile  to  obtaining  reliable  data  for  the  purpose  of 
estaldishing  principles  which  were  not  generally  ad- 
mitted; and  which  have  resulted  in  the  "  Istmietrical 
Bridge  Truss,"  herewith  suljmitted,  for  which  a  patent 
liiis  been  obtained.  The  review,  by  Mr.  Charles  Mac- 
donald.  civil  engineer,  of  the  ])rinciples,  both  mechan- 
ical and  economical,  which  are  involved,  will  be  found 
not  only  interesting  to  the  })rofessional  j'eader,  but  also 
useful  to  bridge  builders  at  large. 

A  Company,  composed  chietly  of  the  younger  mem- 
hersof  the  profession,  has  been  formed  for  the  purpose 
of  introducing   the  new   system  into   use  ;  and  as   it 


':>^'' 


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.  •-, .  •  "•  ' 


•'Nv 


TO    THK    P1JBMC. 


rests  its  claims  for  popular  favor  on  the  solid  basis  of 
practical  experience,  f!;overneil  and  harnioniy,ed  by  ac- 
curate science,  jrreat  confidence  is  felt  in  tbe  advan- 
tages which  will  accrue  to  the  railway  interests  at 
lar<j!;e,  and  to  all  wlkj  desire  a  "  bi^idijjV^to  carrv  them 
safely   over,"    irym   its  licnei^jt^  ^ii^iVdJiV^tioli  and  »'.\- 


tcuded  use 


J.   DUTTON  StEELK, 


Civil   Eiitriiioir, 


\\  -A-  .  —  t 

It'    -.^        :*      -..  > 


4.;'^*!i^ 


V      -  % 


INGOIU'OllATIOK. 


VS 


i 


Tfie  Isometrieal  Truss  Bri(ij!;o  Company  has  .been 
incorporated  under  the  act  of  Assembly  of  the  State 
ol'  Pennsylvania  for  such  purposes  made  and  provided, 
and  the  business  objects  of  said  company  are  fully  set 
fortii    in    the    followinir   extract   from    the    corporate 


agreement 


"  In  order  that  we  may  be  and  remain  a  body  cor- 
porate, under  the  name  and  style  of  The  Isometrieal 
Truss  Bridge  Company,  for  the  purchase  and  sale  of 
United  States  h'tters-patent,  granted  to  J.  Button 
Steele,  on  the  9th  day  of  April,  A.l).  18G7,  for  the  Iso- 
metrieal Bridge  Truss,  and  also  for  the  purchase  and 
sale  of  other  United  States  letters-patent  for  improve- 
ments in  truss  bridges,  the  sale  of  rights  and  granting 
of  licenses  under  said  patents,  or  any  of  them  ;  also 
tor  the  construction  of  bridges,  and  for  enjoying  all 
the  rights  and  j>rivileges  conlerred  by  said  act  of  As- 
sembly," 

The  Board  of  Managers  being  desirous  of  offering 
every  facility  to  parties  wishing  U)  test  the  merits  of 
the  Isometrieal  Truss,  will  furnish  plans  and  estimates 
and,  if  desirable,  secure  the  erection  of  bridges  upon 
this  system  in  any  part  of  the  United  States. 

The  office  of  the  company  is  situated  in  Pottstown, 


[if] 


0 


!t;>^:: 


J  -i 


"J    -^'l'* 


•••  •  1' 


INCORPOnATION. 


Montgomorv  County.  Pennsylvnnia,  and   all   applioa,- 
tiv/ns  for  liconisi^s  shouUl  l»e  a<l(lrossed  to 

W.  D.  Evans,  Ksip, 

Secretary  l-')mi'trh';il  Tnis^  Hri(l>;('  ('nm|i:my. 

Pottstown,  Pii. 


>.'■' 


■1  :'• 

• 
V; 

ii^- 

■•V  ■ 

•  - 

.\--^ 

Tc^pies  oi'  the  aooonijianyinii,  pamphlet  can   he  oh- 

tained  hv  addressinii  the  Seeietar\- as  al)()ve.  \\h(»  will 

•  >-  • 

also  he  in  a  jtosition  lo  lui'iiish  any  other  information 
which  mav  Ic'  desired. 


■  V 


1 

^-V 

■.--.      - ' 

4..,.. ... 


4. 


THE 


ISOMETRICAL  TRUSS. 


Wk  propose  in  the  following  pagos  to  give  a  1)i'iof  de- 
scription of  the  peculiaritieH  of  a  ('(^nibination  in  ln'idge  con- 
stnu'tion,  to  whieh  the  name  of  The  rnonietiieal  Trus?*  ha> 
IxM'ii  applied  ;  and  to  the  proper  nnder8tainling  of  the 
same,  it  will  l»e  nqcessarv  to  look  tsoniewliat  in  detail  into 
the  general  theory  of  strains  in  girders  having  horizontal 
flanges. 

The  writer  approaehes  a  suhjrct,  whieh  in  its  entirety 
involves  so  nuieh  to  interest  the  engineer,  with  eaution. 
ami.  it  is  to  he  hoped,  a  dne  proportion  of  modesty.  The 
ohjeit  ti)  he  attained  is  sim[)ly  truth  ;  and  if  in  this  attenipt, 
a  jii>t  and  seareliing  eritieism  shall  have  demonstratt'(l  the 
fallaty  of  the  views  expressed,  sojiie  good  will  at  leasr 
ha\('  heen  done  in  e.xposing  the  errors  of  a  few  for  the 
henetit  of  all. 

The  general  laws  regnlating  the  aetion  of  strains  in  a 
lieain  supported  at  hoth  ends  and  loatled  throughout  its 
It'iigth,  have  heen  thoroughly  inv«'stigated  hy  some  of  the 
most  emitient  physieists  of  the  day.  And  it  may  not  l)e 
|ti'einatnre  to  assunie  that   the  conclusions  arrived  at  are 


"ri 


)ricnv  as  lollows 


11< 


All  heams  uniforndy  loaded  and  supported  at  hoth  ends. 
iii'c  Nuhjeeted  to  liorizontal  aiid  verti«al  strains.  The  liori- 
zonfal  strain  is  a  ma.ximum  at  tlie  central  cross  section, 
iind  decreases  toward  eacli   [)oint  of  support  in  p»»p(M'tion 


jr 


'J  A; 


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V 


Mi 


f  t, 


ti 


8 


THE    ISO  METRICAL    TRUSS. 


.  •  it   ''■'•. 

>*■  ■■•I  ■.•♦•*■ 

■ '*  V.***-  ■ 

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.".-•*"■•.', 
■•*  *'"  f  •• 

^■^.♦A■ 


■    H 


to  the  ordinates  of  a  parabola.  The  vertical  strains  are 
zero  at  tiie  centre,  and  increase  directly  with  the  distance 
toward  each  point  of  support,  where  they  become  equal  to 
oue-half  the  uniform  load.  In  ca<?e  the  load  is  not  applied 
uniformly,  but  becomes  transient,  as  in  the  case  of  a 
moving  train,  the  horizontal  strain  varies  with  the  pro- 
gression of  the  load,  but  becomes  a  maximum  as  before, 
at  the  centre,  when  the  load  shall  have  covered  the  entire 
beam.  The  vertical  strains  are  so  moditied  as  to  equal 
one-half  the  applied  load  at  each  extremity,  and  gradually 
to  decrease  toward  the  opposite  supports,  becoming  zero 
at  some  point  beyond  the  centre,  and  at  a  distance  from 
it,  varying  according  to  the  proportion  borne  by  the  weight 
of  the  beam  itself  to  the  applied  load. 

These  laws  being  general  in  their  application,  form  the 
basis  of  the  wh(  lo  theory  of  the  action  of  strains  in  hori- 
zontal girders.  We  have,  therefore,  but  to  apply  them  to 
exiritins:  combinatiojis  in  order  to  determine  the  relative 
merits  of  their  construction. 

Beginning  with  the  horizontal  strain,  it  will  be  observed 
that  all  parallel  girders  are  furnished  with  a  top  and  bottom 
chord,  which,  being  horizontal,  are  in  the  best  possible 
position  for  opposing  a  horizontal  strain,  hence  no  theoretic 
advantage  may  be  hoped  for  in  any  radical  change  in  the 
position  of  the  chords.  It  is  not  pretended  that  no  practi- 
cal improvement  can  be  effected  in  the  arrangement  of  the 
details  of  either  the  top  or  bottom  members  of  wood  or 
iron  girders,  but  that  any  such  improvements  can  have 
no  effect  in  diminishing  the  amount  of  strain  theoretically 
called  for  in  such  niembers.  Hence  it  mav  be  assumed,  that 
if  any  advantageous  changes  are  to  be  effected  in  the  pres- 
ent moit  approved  forms  of  girders,  it  will  be  i)roper  to 
look  for  them  in  the  web,  or  vertical  system  wliicli  con- 
nects top  and  bottom  chords. 

In  Fig.  1  let  it  be  assumed  that  a  girder  sustaining  a 
uniform  load,  W  =  2w,  shall  have  been  divided  in  the 
centre,  by  a  vertical  plane  at  right  angles  to  the  plane  of 
the  structure.,  and  that  portion  to  the  right,  removed.  In 
order  to  preserve  the  remaining  portion  in  equilibrium  it 


5^1 


THE   ISOMETRIGAL   TRUSS. 


9 


will  be  necessary  to  exert  a  horizontal  force,  P,  at  the 
centre  of  gravity  of  the  top  chord,  equal  to 

WS 


P= 


8h 


This  value  of  P  is  the  maximum  strain  to  which  the 
chords  are  subjected,  and  being  applied  at  the  point  P,  it 
must  necessarily  pass  through  the  members  of  the  web 


Pig.  1. 


^•)  '■p'^ 


m 


before  reaching  the  point  of  support;  hence  it  becomes  at 
once  apparent,  that  the  particular  arrangement  of  material 
in  tl'is  web  is  a  matter  of  tlie  iirst  importance,  and  we  shall 
soorj  see  wiiercin  some  of  the  present  well-known  combi- 
nations are  defective  in  this  respect.  Since  we  have  to 
deal  with  a  horizontal  force,  the  total  horizontal  section  of 
the  web  from  the  centre  to  the  abutment  must  be  capable 
of  resisting  it;  and  siiice  a  vertical  plate  will  fulfil  this 
condition  with  the  greatest  economy,  theoretically  speak- 
ing, we  should  be  led  at  once  to  the  conclusion  that  a  plate- 
girder  was  th?  best  combination  possible:  but  a  practical 
question  arises  which  places  the  plate-girder  far  in  the  rear 
of  many  other  competitors.  A  vertical  plate  which  would 
be  only  sufticiently  thick  to  resist  the  force  P,  would  buckle, 
hence  vertical  stiffening  members  must  be  introduced, 
which  in  efi'eot  transform  this  species  of  web  into  a  series 
of  posts  and  ties,  the  peculiarities  of  which  we  shall  pres- 
ently examine.  It  may  also  be  mentioned  that  it  would 
be  quite  impracticable  to  introduce  a  thin  plate-web  in 
wooden  bridge  construction,  and  as  this  subject  will  engage 


i": 


•'it-  s 


r:^' 


.■^  ' 


,1'" 

■irr/;   .,• 


'-"•  n 


f. 


10 


THE    ISOMETRICAL    TRUSS. 


the  most  of  our  tinio,  this  papticular  ooinbinatiou  may  be 
left  out  of  tlie  ([ue>5tion. 

Fig.  2. 


Fig.  2  represents  a  case  in  whieli  tlie-load  \\V  is  com- 
municated to  the  top  chord  hy  means  of  vertical  ties: 
should  the  load  he  applied  directly  to  the  top  chord,  stmts 
would  be  substituted  for  tics.  The  force  1\  evidently  tends 
to  pro<liice  rotation  about  the  lowest  jxiints  of  the  vertical 
members,  and  no  resistance  wliatcver  is  opposed  by  tliese 
members  to  such  chaii!j:e  of  form  in  the  structure:  hut  bv 
the  introduction  of  a  diagonal  brace,  as  in  Fig.  ."),  c(jni- 
librium  is  assured,  and  a  wcb-systcm  rendered  comph'te, 
so  far  as  a  uniform  K)ad  is  concerned. 

Should  the  verticals  be  ti'ansfornicd  into  struts,  and  the 
diagonals  into  ties,  as  in  Fig.  4,  the  strains  will  be  simply 
reversed,  but  their  amount  and  the  condition  of  eipd- 
librium  will   remain   the  same.      Although  there  is  really 

Fi)r.  .{. 


no  tlieoretic  advantage  possessed  by  either  ot  tiiese  ar- 
rangements over  the  other,  still  there  are  peculiarities  con- 
nected with  the  kind  of  material  nnide  use  of  in  the  web 


TIIK    ISOMETRICAL    TRUSS, 


11 


iiiid  its  connection  with  tlie  cliord,  sufficient  to  warrant  the 
sy.stonis  bei?i^  called  by  dift'erent  naines.  Fig.  3  presents 
the  main  features  of  wliat  is  tjuniliarly  known  as  the  Ifowe 
Truss,  without  the  eounter-hruce  system;  and  Fig.  4  ap- 
plies similarly  to   the  ohl    l*ratt   hridge,  somewhat    better 

Fig.  4. 


known,  under  a  vsliglit  modification,  as  the  Whipple;  see 
Fig.  10.  Jiut  as  we  liave  seen,  all  vertical  members,  be 
they  ties  or  braces,  involve  an  expenditure  of  material 
which  is  not  in  a  positi<Mi  to  resist  the  action  of  the  force, 
P,  in  tlie  most  economical  manner,  and  a  ccMisidcration  of 
this  fact  in  all  probability  led  to  the  introduction  of  wliat 
is  known  as  the  Warren  girder. 

Fig.  5. 


An  inspection  of  Fig.  5  will  suffice  to  show  tluit  a  diago- 
nal arrangement  of  l)otli  stru.ts  and  ties  is  the  most  ec«)- 
iiomical  i'ov  the  requirements  of  a  web;  ami  it  is  cnpially 
evident,  that  the  aniile  made  bv  tlie  diagonals  with  the 
vertical,  must  have  an  important  bearing  on  the  relative 
•'tllciency  of  diagonal  systems  having  diffi'rent  inclinations. 
It  has  been   clearlv  demonstrated,  both  theoreticallv  and 


i 


'f'J    : 

H  «V7  * 


r'S 


i' ' 


.  >\f  ] 


.'^»««>.,  ♦■■■., 
(^''>l»".^.|rf. 

■>  k  y-  ••■ 

. "'  .'!^v,  ■ 

■,>,■    •-■.*.. I 

"    fe'V      .; 
•  ,.'       •  •    • 

^'^^  ''• 

»:''■  VT»'- 
.  '■     V  /♦•■^ 

(•«     .  «.i 
;i    •.•».' 

■      .  •     '  ♦  h 
'"  *    .•♦.    , 


12 


THE    I80METRICAL   TRUSS. 


practically,  tliat  the  most  economical  angle  with  the  verti- 
cal in  a  triangular  system  18  45",  hence,  if  all  other  things 
were  equally  appropriate,  the  proper  form  for  a  parallel 
girder  would  involve  a  right-angled  triangular  web.  In 
practice,  the  nearest  approach  to  this  is  the  Warren  girder, 
in  which  the  triangles  of  the  web  are  equilateral ;  and 
inasmuch  as  the  relative  economy  of  this  system  is  only 
15  per  cent,  greater  than  the  theoretical  case,  its  many 
practical  advantages  have  earned  for  it  the  first  position. 

The  question  is  now  reduced  to  the  Warren  girder  as 
the  most  economical  form  of  bridge  truss,  having  parallel 
flanges,  and  the  conclusion  is  general  in  its  application 
whether  the  structure  be  of  iron  or  wood.  At  a  subsequent 
period  of  the  investigation  we  shall  see  that  economy  is 
not  the  onlv  advantiiajeous  circumstance  connected  with  a 
triangular  system.  Fig.  6  will  serve  to  represent  what  is 
known  in  this  country  as  the  triangular  truss.  It  differs 
from  the  original  Warren  in  the  introduction  of  vertical  ties 
and  posts,  which  arc  rendered  necessary  in  order  to  afford 
intermediate  points  of  support  between  the  apexes  of  the 
main  triangles.  It  is  evident  that  so  long  as  the  spans  to 
which  the  simple  AVarren  combination  is  ai>plied,  remain 
within  such  limits,  that  the  unsupported  distance  in  each 
panel  shall  not  require  additional  strength  in  order  to  enable 
it  to  sustain  the  distributed  load,  the  vertical  members  in 
Fig.  6  may  be  dispensed  with ;  but  unfortunately  for  the 
success  of  the  system  this  condition  limits  the  extreme 
i^pans  to  a  very  small  margin.  Thus  in  the  case  of  a  rail- 
road bridge  required  to  sustain  a  rolling  load  of  one  and  a 
half  tons  per  lineal  foot,  and  at  the  same  time  to  present 
sufficient  resistance  in  any  one  panel  to  the  weight  of  the 
heaviest  locomotive — say  forty-five  tons — as  applied  at  the 
points  of  contact  of  the  driving  wheels,  we  should  be  ob- 
liged almost  from  the  start  to  introduce  vertical  supports, 
or  otherwise  increase  the  diinensions  of  the  chords  beyond 
what  would  be  required  to  resist  a  horizontal  strain  due 
to  the  whole  load,  since  the  panel  load  per  lineal  foot 
may  greatly  exceed,  possibly  even  double  that  quantity. 
Assuming,  then,  that  the  vertical  members  are  a  practical 


\m 


THE    I80METRICAL   TRUSS. 


13 


Pig.  6. 


Fig.  7. 


'iC' 


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14 


THE    ISOMETRICAL    TRUSS. 


necessity  for  all  eases  witliin  the  reach  of  our  present  in- 
vestigation, and  tliat  their  introduction  was  not  rendered 
theoreticallv  necessary  in  order  to  sustain  tlie  uniform  h)ad; 
in  other  words,  that  they  are  totally  incapahle  of  opposing 
any  resistance  to  change  of  form  in  the  panels;  therefore 
the  material  tlnis  applied  is  a  dead  load  upon  the  useful 
members,  jind  consequently  necessitates  an  additional 
amount  of  material  in  them  to  transfer  this  load  to  the 
abutments.  ISo  much  for  the  question  of  economy;  but 
there  is  another  feature  in  the  use  of  a  single  system  of 
triangles,  which  begins  to  assume  important  proportions  as 
soon  as  the  spans  are  increased  beyond  tlie  ordinary  limits 
now  considered  safe;  which  is  due  to  the  increasing  dimen- 
sions of  the  diagonals  nearest  the  points  of  support.  It  is 
evident,  from  a  consideration  of  Fig.  6,  that  these  mem- 
bers must  be  capable  of  sustaining  a  strain  equal  to  one- 
lialf  the  entire  load  upon  the  truss  multiplied  by  the 
ratio  between  the  diagonal  and  perpendicular.  Now  as  the 
span  increases,  all  other  things  being  }>roportional,  so 
must  this  strain  increase,  and  as  a  natural  conseipience  the 
sectional  area  of  resisting  material ;  and  when  this  area 
reaches  a  certain  limit  it  becomes  necessary  to  introduce 
expensive  appliances  to  fulHl  the  necessary  conditions.  In 
the  case  of  a  Howe  truss,  this  limit  is  reached  at  u  span  of 
one  hundred  and  tifty  feet,  as  will  be  seen  in  the  discussion 
of  that  particular  system  in  a  8ubse(pjent  portion  of  this 
])aper.  Bearing  these  facts  in  mind,  it  is  presumed  that 
we  are  now  in  a  position  to  appreciate  the  introduction  of 
two  independent  systems  of  triangles  in  the  arrangement 
of  the  web.  Referring  to  Fig.  7,  it  will  be  observed  that 
the  diagonals  represented  by  full  lines,  form,  with  the  top 
and  bottom  chords,  a  continuous  Warren  girder,  precisely 
similar  to  Fig.  6,  with  the  exception  of  the  vertical  mem- 
bers. These  members,  we  have  seen,  are  rendered  prac- 
tically necessary  in  the  iiansmission  of  intermediate  load 
to  the  diagonals.  They  add  to  the  dead  weight  to  be  car- 
ried, and  do  not  in  any  way  increase  the  general  strength 
of  the  structure.  Supjtose,  as  in  Fig.  7,  that  these  verticals 
be  dispensed  with,  and  a  second  Warren  system  introduced 


'^ 


•  m 


TUE    ISOMETIUCAIi   TRIJ88. 


15 


as  represented  l>v  the  dotted  (liaj^onals;  it  is  perfectly  clear 
that  the  first  system  will  he  relieved  of  one-luilf  the  load 
which  it  would  otherwise  he  called  upon  to  sustain,  and  that 
this  amount  will  he  taken  ui»  and  curried  to  the  abutments 
hy  the  second  system  ;  at  the  same  time  all  the  practical 
considerations  involved  in  the  vertical  members  have  been 
done  away  with,  and  no  more  sectional  area,  and  con- 
sequent material,  is  recpiired  in  the  diagonals  than  would 
he  called  \'or  in  the  single  full  system  (Fig.  6),  supposing 
it  practically  capable  of  being  adapted  to  a  moving  load 
without  the  use  of  interm  diate  supports.  Hence,  since 
ail  niemberB  which  do  not  assist  in  preserving  the  general 
etiuilibrium  liave  been  removed,  and  the  remaining  ones 
subjected  to  an  equal  share  of  duty  imposed  in  very  nearly 
the  proper  theoretical  direction,  it  may  with  safety  be  as- 
sumed that  a  web  system  involving  a  double  set  of  triangles, 
substantially  as  indicated  in  Fig.  7,  possesses  both  theo-  • 
retical  and  practical  advantages  over  any  other  now  in  use, 
coniinending  it  to  general  introduction. 

Such  are  the  main  features  of  what  has  been  called  the 
Isonietrical  Truss.  The  principles  upon  which  it  is  con- 
structed being  general,  it  is  ecpially  applicable  either  in 
wood  or  iron;  but  as  the  requirements  of  railroad  enter- 
prise in  this  country  demand  more  than  any  other  the 
introduction  of  n  cheap  and  substantial  wooden  railroad 
bridge,  we  shall  devote  the  greater  portion  of  our  remain- 
ing space  to  the  application  of  the  isometripal  principle  to 
wooden  bridge  construction. 

Within  the  limits  of  a  mere  pamphlet,  it  would  be  quite 
impossible  to  attempt  anything  like  an  extended  examina- 
tion of  the  different  kinds  of  bridges  in  use  throughout 
the  country.  Suffice  it  to  say,  that  although  there  are  a 
great  variety,  and  many  of  them  fulfilling  all  the  require- 
ments of  the  original  design,  yet  practically  there  are  but 
a  very  few  systems  acknowledged  to  combine  sufficient 
advantages  t<)  insure  their  continued  adoption;  and  it  is  ' 
with  these  we  propose  to  occupy  our  readers'  attention,  by 
way  of  instituting  a  comparison  between  their  internal 
economv  and  that  of  the  Isometrical  Truss. 


I 


i.  •Vj* 


m 

•Mi 

m 


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^li 


(.^* 


,'*W>.. ■♦'.'# 


!*•*•' 'Ui 


16 


TOR    laOMKTRICAIi   TRUSS. 


The  Howe  bridge  is  probably  tlie  most  extensively 
adopted  wooden  truss  in  the  United  States.  Why  it  is  so 
is  probably  owing  to  a  variety  of  circumstanees,  some  of 
which  may  possibly  not  have  tlieir  origin  in  the  scientific 
merits  of  the  combination;  but  the  fact  of  its  continued 
success  must  l>e  taken  as  evidence  in  its  favor,  and  it  may 
therefore  be  assumed  to  be  one  of  the  best  wooden  bridges 
in  general  use.  The  original  Howe  truss  is  too  familiar 
to  those  interested  in  bridge  construction  to  need  any 
minute  description.  Fig.  9  will  serve  to  illustrate  its  gen- 
eral design.  Previous  to  its  invention,  what  was  generally 
known  as  the  Burr  bridge,  commanded  the  greatest  slnire 
of  attention ;  but  upon  the  introduction  of  railroads,  it 
became  necessary  to  devise  a  structure  possessing  the  ele- 
ments of  stability  under  a  moving  load  to  a  greater  extent 
than  could  be  obtained  in  the  Burr.  The  Howe  patent 
eertaiidy  effected  a  great  improvement  in  dispensing  with 
the  arch  in  combination  with  a  parallel  truss,  and  in  the 
practical  application  of  the  counter  brace,  so  as  to  admit 
of  a  certain  amount  of  rigidity  under  a  moving  load ;  but 
it  soon  became  evident  that  even  this  arrangement  was 
not  adapted  to  spans  of  more  than  one  hundred  and  fifty 
feet,  if  confined  to  the  simple  truss,  by  reason  of  the  dif- 
ficulty in  providing  for  the  increased  vertical  strain  near 
the  points  of  support.  This  will  ap[)ear  more  clearly  when 
a  particular  case  is  subjected  to  the  test  of  calculation. 
Suppose  a  single-track  railroad  bridge  to  be  required  of 
one  hundred  and  fifty  feet  span,  the  weight  of  a  maximum 
applied  load  being  taken  at  one  and  a  half  tons  per  lineal 
foot,  and  the  weight  y>t'  structure  at  a  half  ton,  making 
a  total  load  of  two  tons  per  lineal  foot  of  bridge.  The 
total  weight  to  be  sustained  by  the  bridge  will  be  three 
hundred  tons;  and  since  very  nearly  one-half  of  this  strain 
is  transmitted  to  the  abutments  through  the  web  on  each 
side  the  centre,  it  will  be  necessary  to  proportion  the  rods 
and  braces  in  the  last  panels  to  resist  such  strain.  The 
exact  figures  are  one  hundred  and  forty  tons  for  each  end 
of  the  bridge;  taking  a  divisor  for  safety  of  five  tons 
per  square  inch  of  section  in  iron,  we  require  twenty-eight 


THE    180METRICAL    TRUH8. 


17 


8qnui'C  iiK'lics  in  all  for  (lie  end  rodn.  In  general  i>ractice 
there  are  six  rods  in  the  last  panels,  or  three  I'or  eaeh  truss. 
Dividing  twenty-eight  by  six  we  have  4,^^  sqvuire  inches 
tor  each,  or  2". 44  diameter  for  round  iron.  The  strain  upon 
the  braces  will  of  course  be  increased  by  the  ratio  of  the 
diagonal  to  the  per[>endicular ;  but  these  members  l)eing 
in  wood,  <lo  not  oppose  the  same  practical  ditftculties  to  an 
increase  of  section  as  in  the  case  of  tlie  vertical  rods.  Round 
iron  of  this  size  2".44  in  diameter,  is  practically  (piite  as 
large  as  it  is  desirable  to  employ;  and  any  increase  in  the 
span,  carrying  witii  it  as  it  does  an  increased  demand  for 
resisting  material,  makes  it  necessary  to  introduce  expen- 
r-ive  a}4>liance8  for  the  purpt>se  of  increasing  the  number 
rather  than  the  diameter  of  end  members. 

It  may  be  insisted  that  the  assumed  rolling  load  of  one 
and  a  half  tons  per  lineal  foot  is  excessive,  and  in  justiKca- 
tion  of  the  objection  reference  might  be  made  to  excep- 
tional cases  wherein  the  llowe  truss  liad  been  used  for 
railr(»ad  purposes,  on  somewhat  greater  spans  and  much 
less  dimension  of  rods;  l)ut  we  are  compelled  to  adopt  this 
seemingly  high  limit,  from  a  ^u-actical  knowledge  of  the 
extreme  load  which  is  sometimes  brouii:ht  to  bear  on  the 
bridges  of  tlie  Philadelphia  and  Heading  Railroad,  and 
from  a  desire  to  maintain  the  practice  of  bridge  building 
fully  up  to  the  requirements  of  the  age.  In  order  to  over- 
couie  the  ditHculties  above  mentioned,  what  was  called  an 
improvement  in  the  llowe  truss  was  effected  by  falling 
back  upon  the  discarded  arch  of  the  I'urr  in  combination 
with  the  llowe;  a!id  this  new  arrangement  has  been  at- 
tended with  a  measure  of  success;  but  that  there  are  serious 
objections  to  the  combination  of  the  arch  and  truss  for  rail- 
road })urposes  no  practical  bridge  builder  will  deny;  and 
it  is  onlv  because  no  better  svstem  presents  itself  that  the 
Improved  Howe  retains  the  Held.  This  is  written  with  a 
somewhat  intimate  knowledge  of  the  peculiarities  of  other 
wooden  bridges,  which,  in  theory,  may  possibly  lay  cUiim 
to  merit.  For  example,  the  McCallum  Intlexible  Arch 
Truss,  as  explained  and  illustrated  by  the  inventor,  is  one 
of  the  most  beautiful  of  wooden-bridge  combinations,  and 


iX. 


>; 


:\7 


:^:3 


s  »•,■ 


hi. 


i-tJ 

m 


t 


111   '^'-^ 

MB'*'.  .•■.'•; 


18 


THE    ISOMKTRICAIi    TIIU88. 


;•■■"  .VJ 

....  '^     ..  •■» 

_  i^*"-^".  ■■  "••  • 

F  "'^>^  .V 


(it-  •  i 


yet  wlion  siibjoctod  to  the  test  of  time  it  is  utterly  iniprac- 
ticablo:  witii('j*«  the  McCallinii  bridge  earrying  tlie  North- 
ern Central  JIaiUvaj  over  the  Susquehanna  River  above 
Ilarrirtburg,  which  has  just  been  replaecd  by  an  Improved 
Howe. 

Next  to  tbe  Howe,  and  in  many  particulars  superior  to 
it,  is  the  VV^hippie  Truss,  a  inoditication  of  the  Pratt  (Fig. 
10),  whicli  we  have  seen  is  but  a  substitution  of  the  diago- 
nal rod  and  vertical  jtost,  for  the  vertical  rod  and  diagonal 
brace  of  the  Howe.  The  practical  advantages  possessed 
by  this  truss  over  the  Howe,  consist  for  the  most  part  iu 
a  saving  of  material  over  the  piers  in  case  of  a  through 
bridge,  and  in  the  first  two  panels  beyond  the  pier  in  the 
case  of  a  deck  bridge;  there  are  also  some  minor  a<lvan- 
tages  in  the  connection  of  the  posts  and  rods  with  the 
chords,  but  the  important  fact  remains  the  same,  that  the 
end  rods  and  braces  must  sustain  the  entire  load  upon  the 
truss,  and  of  course  the  same  ditticulties  as  were  experi- 
enced in  the  Howe  truss  for  long  spans  may  be  expected 
in  the  Whipple.  Other  systems  might  be  mentioned,  pre- 
senting many  points  of  interest  to  the  practical  observer, 
but  as  the  two  already  noted  cover  as  much  ground  and 
are  probably  better  understood  than  any  other  systems  i'.i 
general  uso^  we  shall  nut  weary  the  re  der  with  further 
examinations.  Both  these  trusses  involve  diagonal  and 
vertical  members  in  their  web,  and  eonsecpiently  are  com- 
pelled to  furnish  and  carry  much  additional  material  above 
what  is  absolutely  recpiired  in  the  transmission  of  the  load 
to  the  points  of  support,  if  efJected  by  the  most  economical 
lines;  and  no  amount  of  perfection  in  workmanship  or  in- 
genious appliances  can  by  ai\y  possibility  retnove  the  de- 
fect, since  it  is  inherent  in  the  general  design  and  must  con- 
tinue as  a  characteristic  of  the  structures. 

Again,  we  have  seen  that  the  liniiting  span  in  eacli  of 
tbe  above  systems  is  about  the  same;  and  here  let  it  be 
thoroughly  understood  that  the  ternk  limiting  span  in  this 
connection,  is  not  intended  to  apply  to  the  theoretical  limit 
to  which  a  wooden  girder  of  either  pattern  may  be  built, 
but  the  practical  limit  beyond  which  the  ordinary  sizes  ot 


'''£^«» 


THE    IHOMKTRirAL   TRUSS. 


19 


round  iron  in  ifononil  xit^c  become  too  small  tor  the  nuMn- 
liei'H  of  the  weh  nearest  the  points  of  support;  beyond  tliis 
tlie  cost  of  vvorkniansliip  "becomes  severe. 

A  few  years  aifo,  spans  of  one  liundred  and  fifty  feet 
Miii^lit  possibly  have  expressed  the  limit  of  enjLfineennii^  re- 
(|nirements,  ])ut  it  must  not  be  assumed  that  in  such  an  ad- 
vanced au^e,  wheii  railroads  are  bein^  projected  und  built 
aerosol  continents  (arid  eve»i,  possibly,  under  the  very  ocean 
itself),  that  such  contracted  facilities  will  long  be  tolerated. 
And  it  behooves  the  engineer  to  look  well  to  bis  laurels 
that  the  profession  of  his  choice  is  not  sutfered  to  lag  be- 
liind  in  the  march  of  improvement. 

In  the  discussion  of  Fig.  7  it  lias  been  shown  that  the 
double  diagonal  system,  involved  in  the  Isometrical  Truss, 
possesses  greater  claims  to  rigidity  and  economy  than  any 
other  web  system  in  use,  and  also  that  in  the  distribution 
of  the  strains  upon  the  web  members  a  great  economy  of 
sectional  area  was  effected;  thus,  for  example,  the  rods  and 
braces  a  h,  c  (/,  </,  r/  h  (Fig.  7),  are  called  upon  to  perform  half 
the  work  of  the  braces  and  ties  of  the  other  systems,  hence 
their  sectional  area  need  not  exceed  one-half  that  of  the 
corresponding  members  in  the  others.  In  order  to  apply 
to  this  questioji  of  economy  the  more  exact  test  of  figures, 
we  have  calculated  the  strains  upon  tlie  three  systems,  as 
represeoted  in  Figs.  8,  9,  and  10,  and  liaving  the  common 
data  as  below : 

HINOLE-TRACK   THROUGH    BRIDGE. 

Span,  lcr>.2  feet. 

Centre  to  centre  of  chords,  20.438  feet. 

Lcngtii  of  diat^onals,  23.0  feet. 

Lenjjjth  of  each  panel,  11.8  feet. 

Weight  of  bridge,  half  ton  per  lineal  foot. 

Rolling  load,  one  and  a  half  torn?  per  lineal  foot. 

Number  of  panels*,  14. 

The  numbers  attaclied  to  tlif  ditterent  members  repre- 
Hont  the  maximum  strain,  in  tons  of  two  thousand  pounds, 
to  which  tiiey  can  possibly  be  subjected  Oy  the  action  of 
the  moving  load  and  the  weight  of  the  bridge  itself;  and 


*:.vi 


4. 

t,  •'I'll' 


iM-' 


'C 


if 


"TT^^^l^^^^aarsBT- 


»«:.' 


'*r 


i      ■  t  ^  ■  ■       ■"' 


I' 


(I     -4  1     ^W  ...    *  ■ 


■  1 
■J- 


'=!-*v 


iw* 


..■■'■■V 


Wf| 


20 


THE    ISOMEiUICVL   TRUSS 


Fix.  8. 


Fig.  0. 


"^  ■    /j 


THE    ISOMETUICAL    TRUSS, 


21 


Fig.  10. 


Fi^.  11. 


1 

m 


I  •»'•• 


•*"' 


t^A, 


^ 


J  '  V, 


..^.iiWiiU. 


1/ '!■  ill  ''V  <*■  ^'  "■'** 
Vrf  i;  <.^-  *'  •  ■ 

^^■-  ■ 

lii^i.. .''♦:■„ 


22 


THE    ISOMETRICAL    TRUSS. 


.*^' 


.U'V-,  ••  ' 


•^vrV- 


r.'-. 

•   V 
Iv'--.  ! 
I*   *. 

■■■«»-  ■■• 
'^liJ^'^    ■■,'■ 

''■"■  ^.'^  •    , 
,■"•  •*..•■*.• 

•  if'     ■-'•••. 


"rfH 


t'. 


w^ 


rs: 


the  signs*  +  or  —  represent  compression  and  tension  re- 
spectively. It  will  be  observed  that  the  central  members 
of  the  Isometrieal  Truss  are  subjected  to  both  +  and  — 
strains.  This,  of  course,  is  to  be  understood  as  due  to  the 
counteraction  of  a  moving  load,  and  is  provided  for  in  the 
other  systems  by  members  introduced  expressly  for  the 
purpose.  In  the  Howe  truss  we  have  indicated  the  coun- 
ter braces,  which  are  not  theoretically  required,  by  dotted 
lines;  they  are  always  introduced  in  practice,  but  are  not 
required  uidess  it  be  to  hold  the  angle  blocks  against 
which  the  main  braces  abut.  The  method  by  which  the 
strains  were  calculated  in  Figs.  9  and  10  is  too  well  known 
to  need  any  explanation.  In  Fig.  8  each  system  of  tri- 
angles was  treated  independently,  and  the  load  upon  each 
panel,  including  the  weight  of  panel  itself,  was  supposed 
to  be  applied  at  the  point  of  each  panel  division.  This  is* 
not  strictly  true  in  practice,  since  that  portion  of  the  load 
which  is  fi^ed  will  be  partly  sustained  at  the  top  of  each 
division  by  the  brace,  but  the  error  in  effect  is  insigniti- 
cant. 

It  may  be  interesting  to  notice  that  the  effect  of  a  change 
of  load  from  the  bottom  to  the  top  chord  would  be  to  trans- 
fer the  amount  of  the  minus  strains  with  the  signs  changed 
to  the  compression  members  in  each  panel,  and  vice  versa. 
The  first  thing  that  attracts  attention  in  the  examination 
of  these  figures  is  the  great  disparity  between  the  strains 
upon  corresponding  mend)ers  of  the  Isometrieal  Truss  and 
the  other  two;  but  in  order  to  reduce  these  figures  to  a 
tangible  shape  it  will  be  necessary  to  determine  the  actual 
amount  of  material  required  to  resist  the  strains,  in  each 
system,  and  then  determine  the  relative  percentage.  This 
may  be  effected  with  sufficient  accuracy  for  our  present 
purpose  by  adding  the  total  strains  having  the  same  signs 
in  each,  and  multiplying  the  result  by  the  theoretic  length 
of  the  members  corresponding.  Thus  the  total  minus 
strain  in  the  Isometrieal  including  counter  svstem  is  444 
tons;  assuming  a  divisor,  for  safety,  of  5  tons  per  scjuare 
incli  for  wrought  iron,  this  will  re<|uire  HS^^f^  square  inches, 
which,  multiplied  by  23.6  feet,  the  lengtli  of  a  diagonal, 


■*^9*mi 


THE     ISOMETRICAL   TRUSS. 


23 


(^ 


'1M 


jiiid  rlivided  by  144,  gives  14  ,«^  cubic  feet  of  wrongbt  iron 
for  one-half  the  bridge  In  tlie  same  way  the  amount  of 
iron  in  the  Howe  trusts  is  found  to  be  17/jj  cubic  feet,  and 
in  the  Whipple  20,»5  cubic  feet.  For  the  plus  strains  wood 
has  been  assumed  as  the  resisting  medium,  and  a  corre- 
sponding divisor  for  safety  of  one-half  ton  per  square  inch. 
The  results  for  compression  are — 

Isometrical, 138       cubic  feet. 

Howe, 258^5 

Whipple, 166^^5  " 

By  comparing  these  quantities  with  each  other  and  ar- 
ranging them  according  to  their  relative  position  we  have, 
calling  the  Isometrical  100 — 

Table  I. 


&r 


Name. 

Rplatifefcon- 
omy  of  Iron. 

R»'lativ«  econ- 
omy of  wood. 

AvfrauB  pcon- 
oroy. 

— 

Remarks. 

Isometrical  Tru.ss,    .     . 

100 

100 

100 

Web  sys- 
tem alone 

Whipple          "        .     . 

U3 

120 

13U 

in  each 
case. 

Howe                «»        .     . 

122^ 

187 

154/, 

Thus  it  appears  that  the  Whipple  involves  an  expendi- 
ture of  43  per  cent,  more  iron  than  the  Isonietrical  in  the 
web  system,  and  20  per  cent,  more  wood,  and  the  Howe 
truss  22|  per  cent,  more  iron  and  87  per  cent,  more  wood. 
It  is  possible  that  some  slight  variation  of  these  figures 
might  be  obtained  by  a  different  arrangement  of  the  num- 
ber of  panels,  but  nothing  in  general  practice  will  warrant 
a  more  favorable  expectation  for  either  the  Whipple  or 
Howe. 

Again  alluding  to  the  subject  of  ordinary  limiting  span, 
we  find  the  end  diagonals  (Fig  8),  subjected  to  a  strain  of 
9')  tons.  Taking  as  before,  a  divisor  of  5,  this  will  require 
IH  s(piare  inches  of  cross-section,  and  as  there  are  three 
rods  in  each  panel,  or  six  in  all,  we  have  for  each  3.16  square 
inches,  or  2"  round  iron.  Figures  9  and  10  give  for  cor- 
responding ntembers  153  and  152  tons,  which  for  the  same 
divisor  and  number  of  rods,  call  for  2",Yo   ''<>"'>d  iron, 


#3 


.•I.  V 


(^f■ 


t^H 


'^y 


\^:AtiJir^.jL.'i. 


-Zi  .S-  JjTTJlS-y-lS 


m 


''37  ■•■■■  r'     Jl 


24 


THE    ISOMETRICAL   TRUSS. 


rii'i..,"".,-^ 


._♦  >,#.^.  -' 

41  >  -ft*  «J 

■•"■•/?■;" 
■J'  ■    '     '. 


"<>i 


4i  . 


'«** 


! 

s 

I'l 

i 

1 

.4 

which,  as  we  have  soon,  is  bevond  the  ordinary  limit  of 
size;  while  on  the  other  hand,  the  Ison.etrical  mityht  be 
increased  very  considerably  in  span  betore  the  limiting 
size  of  2"h  would  be  renuired. 

The  above  results  are  but  the  reduction  to  figures  of  the 
general  conclusions  arrived  at  in  the  earlier  part  of  the 
discussion,  and  altboiigh  they  are  confined  to  but  three 
systems  of  wooden  luidi^os,  it  is  believed  that  sufficient 
evidence  has  been  deduced  to  establish  the  claim  of  the 
Isometrical  Tru-'ss  to  the  first  position  among  structures 
of  this  class. 

Although  our  remarks  have  been  confined  to  two  promi- 
nent types  of  wooden  bridges,  by  way  of  comparison,  the 
same  conclusions  will  apply  to  structures  in  iron  possessing 
the  same  general  features.  Thus,  for  example,  in  the  dou- 
ble system  of  wrotiiiht  iron  columns  and  diaironals  made 
use  of  bv  Mr.  Linville  in  his  ma<rnificent  bri(lire  across 
the  Ohio  at  Steubenville.  involving  a  central  span  of  320 
feet,  and  still  later,  in  the  (\»nneetinir  Railroad  bridir^*  across 
the  Schuylkill  at  IMiiladelpbla  of  250  feet  span,  we  find 
the  vertical  members  of  the  Whipple  repeated,  to  which, 
in  a  great  measure,  may  be  attributed  the  ex'cessive  ex- 
penditure of  3n  p''r  cent.  <»f  material  o/er  the  Isometrical. 
It  is  true,  however,  that  the  introduction  of  a  double  sys- 
tem permits  an  economy  of  material  in  the  web,  over 
what  it  would  be  iiracticable  to  eflect  in  the  siuirle  system 
of  Fig.  10,  by  enabling  the  angle  between  the  diagonal 
and  vertical  to  be  increased  to  its  most  econojnical  limit, 
or  55°  very  nearlv.  Still  another  advantajre  is  effected  bv 
this  double  system,  the  importance  of  which  has  been  fully 
diseussed  in  the  previ()useom|>arisons,namel3',  the  increased 
facilities  for  long  spans,  and  in  this  respect  the  Isometrical 
has  only  a  very  slight  advantage  over  tlie  Linville  t:  tiss. 
For  the  purpose  of  instituting  an  economic  comparisoti 
between  these  two  bridges  in  iron,  a  Linville  truss  has 
been  arranged  and  calculated,  jis  in  Fig.  11,  having  the 
same  general  data  as  before  mentioned;  and  one  word  in 
this  regard,  befor<'  passing  to  a  comparison  of  figures.  In 
order  to  prevent  a  multiplicity  of  <liagrams,  the  same  height 


THE    ISOMETillCAL   TRUSS. 


25 


of  truss  and  number  of  panelm  were  assumed  for  the  Lin- 
ville  as  the  Isometrieal,  so  far  as  heipjht  is  concerned,  any 
change  vvouhl  effect  bot^i  equally  in  the  comparison,  but  it 
is  possible,  that  in  a  practical  case,  Mr.  Linville  might  in- 
troduce a  ditterent  arrangement  of  panels  in  order  to  bring 
the  diagonal  angle  more  nearly  equal  to  its  economical 
limit  of  55°.  But  it  is  believed  that  any  slight  variations 
of  this  nature  could  not  affect  the  result  materially. 

The  niethoa  of  ju'ocedure  in  deterininitig  the  following 
percentages  is  precisely  similar  to  that  which  was  adopted 
in  the  previous  cases,  namely,  by  comparing  the  absolute 
quantities  of  material  as  determined  from  the  strains 
marlced  on  the  diagrams. 


Table  IT. 


Name. 

RplHtlTe  ernn 
oniy,  Tt-iisiiin. 

Itflativp  econ- 
omy, Com- 
prf88ion. 

ATerage  econ- 
omy. 

Remarks. 

Isoinotrical  Truss.    . 
Linvillo           "         .     . 

100 
158 

1(10 

77 

100 

119 

Web    sys- 
tem alone 
in      each 
case. 

The  result,  as  will  be  observed,  is  not  by  any  means  so 
startling  as  in  the  previous  c.nses,  since  there  is  an  absolute 
saving  of  (28)  per  cent,  of  material  in  the  compression 
members  against  a  loss?  of  58  per  cent,  in  tension,  the  dif- 
ference, or  10  per  cent.,  is  the  pn)p()rtion  of  inofeased  ex- 
penditure in  the  web  svstem  of  the  Linville  truss  over 
tliat  of  the  I-sometrical. 

The  tact  that  bridges  built  uj>on  this  princifJle,  have 
fully  realized  the  most  sanguine  expectations  of  their  pro- 
jectors, and  the  public  generally,  is  no  argument  in  favor 
of  the  theoretical  accuracy  of  their  design  nor  of  the  prac- 
tical economy  of  their  Tonstruciion,  but  rather  that  they 
are  the  best  applications  of  the  principles  upon  whicli  they 
are  constructed  to  practice,  and  most  certainly  entitle  their 
originator  to  a  very  high  rank  in  the  special  branch  of  the 
profession  to  which  he  has  devoted  his  attention. 

It  is  not  by  any  means  claimed  for  the  Isometrieal  Truss 
tliat  it  is  a  perfect  c<mibination,  and  conse(piently  superior 


m 


'a 


-  it 


■*>j 


.^ 


•^  .■ 


».»•,!(« 


A  , 


1 

I. 


26 


THE    ISOMETRIOAL   TRUSS. 


to  any  and  every  other  now  in  use,  as  the  writer  is  well 
aware,  that  in  tlie  almost  infinite  variety  of  eircumstances 
which  the  engineer  is  called  upon  to  provide  tor,  the  par- 
allel girder  must  frequently  he  laid  aside  for  other  and 
more  appropriate  appliances.  But  of  this  we  feel  compelled 
to  insist  that  it  is  t!ie  most  practical  application  of  the  true 
theory  of  the  action  of  forces  in  a  parallel  girder  now  in 
existence. 

Hitherto  our  comparative  investigations  liave  been  con- 
fined to  the  two  questions,  of  economy  of  construction 
and  adaptability  to  long  spans,  and  in  those  we  have  shown 
the  Isometrical  Truss  to  be  founded  on  sound  principles. 
But  there  are  other  considerations  involve<\  in  the  discus- 
sion, which  are  of  great  importance  in  arriving  at  a  proper 
conclusion,  and  they  may  be  generally  expressed  by  the 
terms  compensation  and  adjustability. 

.  The  iirst  is  understood  to  be  the  abilitvof  a  structure  to 
adapt  itself  to  the  varying  circumstances  of  a  changeal)le 
temperature  and  load;  and  tlie  second,  relates  to  the  facil- 
ities presented  for  restoring  it  to  a  normal  condition,  ren- 
dered necessary  by  the  deteriorating  influences  of  time.  In 
regard  to  compensation,  the  very  nature  of  the  construction 
of  the  Isometrical  Truss,  involving  as  it  does,  equal  equi- 
lateral triangles  in  all  directions,  involves  perfect  uniform- 
ity of  action  under  either  circumstance  of  change  of  tem- 
peratures! or  load,  hence  there  can  be  no  distortion  of 
figure  within  the  limits  of  a  maximum  load,  and  the 
truss  is  therefore  practically  rigid. 

In  orAr  to  illustrate  the  necessity  of  proper  adjusting 
facilities  in  a  truss,  the  following  diagram  shows  the  change 
which  takes  place  in  any  one  panel  of  a  parallel  girder, 
under  the  action  of  a  load  or  the  deterioratinir  influence 
of  time. 

A  deflection  below  the  normal  position  of  the  truss 
manifestly  causes  a  compressive  strain  on  the  diagonals 
a,  a,  a,  and  a  tensile  strain  upon  h,  b,  b.  So  long  as  this 
strain  does  not  exceed  the  elastic  litnit  of  the  material, 
these  members  will  of  themselves  force  a  return  to  the 
normal  immediately  upon  the  removal  of  a  loud,  and  sii»je 


"I*', 


m 


THE    ISOMETRICAL   TRUSS. 


27 


Fig.  12. 


they  are  placed  in  tlie  direction  in  which  the  panel  changes 
form,  ihey  will  roqnire  to  exert  less  eft'ort 
in  accomplishing  the  ohject  than  in  any 
other  position.  For  the  same  reason,  if 
the  truss,  after  a  lapse  of  time,  shall  have 
hecome  permanently  deflected,  we  have 
but  to  screw  up  the  diagonals  h,  6,  b,  to 
secure  entire  adjustment.  The  manifest 
superiority  of  this  system  over  others  in- 
volving an  adjustment  through  vertical 
members,  is  now  apparent,  since  the  best 
possible  way  to  a-)[>ly  force  to  b,  h,  6,  in  • 
order  to  shorten  them,  is  in  their  own  line 
of  action,  and  if  it  becomes  necessary,  as 
in  the  Howe  truss,  for  example,  to  shorten 
the  vertical  rods  in  order  to  effect  the 
same  result,  much  greater  expenditure  of 
force  is  required,  and  this  accounts  tor  the 
fact  admitted  l)y  all  practical  biidge  build- 
ers, that  when  a  Ilowe  truss  becomes  j)er- 
nuiJiently  deflected  below  the  horizontal, 
expensive  false  works  are  necessary  to 
efl'ect  a  restoration.  We  shall  have  occa- 
sion to  notice  the  truth  of  this  observation 
in  the  discussion  of  some  experiments 
up(in  models. 

During  the  earlier  consideration  of  some 
of  the  questions  treated  of  in  this  paper, 
the  writer  was  fortunate  enough  to  secure 
the  perusal  of  an  ably-written  analysis  of 
four  well-known  iron  bridges  by  Mr.  0. 
iShaler  Smith;  and  feeling  that  the  conclu- 
sions therein  arrived  at  would  warrant  a 
more  expended  range  of  favorable  comparison  for  the  Iso- 
nietrical  Truss,    t 


Synop 


(^ 


III) 


has  been  transferred  direct.  And  it  is  perhaps  needless  to 
add  that  .\[r.  Smith's  well-established  reputation  is  a  sutK- 
cient  guarantee  for  the  correctness  of  results  thereiii  con- 
tained. 


f? 


r 


•ft-{ 


1*1'  M 


iii 


4^1 


..  .  ■'.     >jdA^  ..jv  '  '    ^'AsSsilL^i&L  Aud^ 


28 


THE    ISOMKTKKAr-    TRUSS. 


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THE    I80METRICAL   TRUSS. 


29 


The  triangular  truss  mentioned  in  tins  connection,  is 
[(recisely  the  same  as  has  been  noticed  in  Fig.  6,  and  the 
marked  su[»eriority  of  the  Isometrical  system  over  this  ar- 
rangement has,  we  trust,  been  fully  established.  From 
our  previous  discussion  of  the  provisions  for  coi^pensation 
and  adjustment  in  this  system,  we  feel  compelled  to  assume 
the  responsibility  of  placing  it  absolutely  Hrst  in  every 
respect  except,  perhaps,  its  adaptability  to  all  spans  and 
positions;  and  in  this  particular  an  equality  is  claimed 
with  the  Fink. 

The  original  application  of  the  Isometrical  principle  to 
iron  bridge  construction,  is  believed  to  have  been  made  on 
the  riiiladelplna  and  Reading  Railroad,  in  the  year  1863. 
A  series  of  experimental  trusses  of  GO  feet  clear  span  were 
constructed  by  the  patentee  in  the  sliops  of  the  company  at 
Pottstown,  and  afterwards  put  up  at  different  points  on  the 
line,  where  they  were  subjected  to  the  most  severe  trials  ca- 
pable of  being  apjdicd  in  practice,  and  the  results  were  in  all 
cases  highly  satisfactory.  These  same  bridges,  it  may  I; .' 
remarked,  are  now  carrying  the  heavy  trade  of  this  well- 
known  railroad  without  the  sliglitest  indication  of  weak- 
ness or  the  expenditure  of  a  dollar  for  adjustment.  It  was 
not,  however,  until  the  summer  of  1865  that  any  attempt 
was  nuide  to  a[»i)ly  these  principles  to  wooden  bridge  con- 
struction, but  the  results  then  obtained,  although  throuirh 
the  medium  of  experiments  upon  models,  were  full  and 
conclusive.  A  brief  description  of  these  experiments  may 
not  l»e  uninteresting  to  our  readers.  The  lirst  model  was 
constructed  in  the  IMiiladelphia  and  Reading  Railroad 
Company's  shops,  at  I'ottstown,  on  the  general  design 
represei»ted  in  Fig.  7-  It  consisted  of  a  single  truss  of  :25 
feet  span,  having  its  parts  propoi'tioned  upon  a  scale  of  J 
the  size  of  a  bridge  200  feet  span,  but  no  p^rovision  was 
made  lor  a  system  of  counterbracing.  After  placing  the 
niodwd  tirndy  on  its  supports,  and  contining  the  upper 
chord  so  as  to  prevent  buckle,  a  uniform  load  of  castings* 
was  aj»plied  to  the  lower  chord, of  3500  pounds,  i)roducing 
a  deflection  of  1^  inches ;  but  in  order  to  test  the  eliect  of 
a  want  of  counterbracing,  the  t..«,tings  from  one-half  the 


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I* 


30 


THE    ISOMETRICAL   TRUSS. 


truss  were  rernoved'jnul  it  was  noticed  that  tlic  braces  near- 
est tlie  middle  hecame  lo(3se.  In  order  to  remedy  tliis 
evil,  the  entire  load  was  removed  and  iron  tie-rods  intro- 
duced alongside  of  the  braces,  and  in  onler  to  make  room 
for  them,, their  entire  section  had  to  be  cut  out  of  the 
chords,  top  and  bottom.  After  this  had  been  accom- 
plished, an<l  all  the  parts  brought  to  a  proper  bearing, 
the  uniform  load  of  3500  pounds  was  again  applied,  but, 
singularly  enough,  the  truss  scarcely  deflected  \  of  an 
inch,  and  more  remarkable  still,  the  rods  which  had  lately 
been  introduced  in  the  direction  of  the  main  braces,  were 
evidently  carrying  a  certain  proportion  of  strain.  This 
was  manifest  to  all  who  witnessed  tlie  experiment,  both 
from  the  tightness  of  the  rods  themselves,  and  the  crushed 
fippearance  of  the  wooden  washers  upon  which  their  bolt 
heads  rested.  Feeling  confident  that  some  new  element 
of  strength  had  been  introduced,  it  was  determined  to 
gradually  increase  the  applied  load  to  the  breaking  point, 
if  possible,  in  order  to  ascertain  the  conditions  of  failure; 
but  after  occupying  all  the  available  space  on  the  chords 
with  castings  to  the  weight  of  7000  pounds,  and  noting  a 
deflection  of  only  one  inch,  it  was  thought  best  to  test  the 
effect  of  time  upon  the  structure  under  such  a  load,  and  in 
tlie  mean  time  prepare  a  Howe  truss  of  same  dimensions 
for  comparison.  This  was  accordingly  done,  but  in  order 
to  make  this  truss  complete  by  the  introduction  of  a  coun- 
terbrace  at  once,  it  was  found  impracticable  to  retain  more 
than  two  pieces  in  the  main  braces  '>f  each  panel,  while 
the  Isometrical  retained  three,  thus  giving  it  a  superiority 
of  one-third  the  amount  of  brace  tind>er;  in  all  other  re- 
spects both  trusses  were  alike,  so  far  as  the  nature  of  their 
designs  would  permit.  Although  this  single  exception  is 
to  be  regretted,  it  is  not  believed  that  the  vast  difference 
between  the  results  obtained  can  be  attributed,  even  in 
part,  to  it,  since  the  tensile  system  was  the  same  in  each, 
and  the  Howe  truss  had  not  liad  ita  chord  section  de- 
creased one-ninth  by  the  ad<lition  of  brace-rods,  as  in  the 
case  of  the  first  model.  After  being  set  up  in  place  close 
to  its  fellow,  this  truss  was  loaded  in  the  same  manner  as 


l/t^  *•  •^ 


'the  isometrical  truss. 


81 


the  other,  witli  castings  accurately  weighed  for  the  pur- 
po8e,biit  upon  the  application  of  3400  pounds,  it  was  ob- 
served that  the  bottom  chord  had  become  so  badly  bent 
near  the  abutments,  and  depressed  in  all  1|  inches  in  the 
centre,  that  a  complete  rupture  was  feared  on  the  instant, 
and  to  prevent  entire  loss,  supports  were  introduced  under 
the  chord  until  such  time  as  the  weight  could  be  removed. 
A  subsequent  examination  of  the  model  showed  the  bolt 
heads  connected  with  the  vertical  rods  badly  crushed  into 
the  chords,  although  the  bearing  was  complete  throughout, 
and  the  original  camber  of  the  chords  had  been  perma- 
nently changed  into  an  undulating  curve,  similar  to  the 
cross-section  of  a  corrugated  plate.  The  writer  attempted 
to  restore  the  structure  to  its  original  position  by  tighten- 
ing the  vertical  rods,  but  the  most  violent  exertion  with  a 
heavy  wrench  failed  to  accomplish  the  object.  In  fact, 
the  structure  was  in  a  condition  to  require  the  introduc- 
tion of  false  works  on  a  small  scale. 

Turnini;  to  the  Isometrical  Truss  with  its  load  of  7600 
lbs.  still  upon  it,  a  close  inspection  failed  to  detect  the  least 
indication  of  weakness,  except  perhaps  the  crippled  condi- 
tion of  the  temporarily  introduced  wooden  washers  attached 
to  the  brace  tie-rods,  the  curve  of  deflection  in  the  chord 
was  slightly  concave  at  the  centre;  but  at  the  ends  wbere 
the  curvature  in  the  Howe  was  most  severe,  there  was  no 
indication  of  any  depression  whatever.  The  eftective  area 
of  this  chord  at  the  centre  was  four  square  inches;  the 
height  from  centre  to  centre  of  choids,  3.06 /^  Hence  by 
the  equation  of  moments  : 


P= 


S  W     25  X  7600  lbs. 


8  h 


8  X  3.05 


:7787  lbs. 


But  for  a  divisor  of  1000  lbs.  per  square  inch,  this  would 
cull  for  7|Yg  square  inches  of  chord  section,  or  nearly 
double  the  amount  actually  existing.  We  are  well  aware 
that  from  the  circumstances  connected  with  the  construc- 
tion of  models,  it  is  impracticable  to  extend  any  numerical 
results  which  may  be  obtained  from  experiments  upon 
them,  directly  to  the  full-sized  structures  which  they  may 


m 
m 

m 
m 

•'"»,4  - 


m 


32 


THE    IHOMETRICAL   TRUSS* 


I   •■    ■•■■    \  V. 
'  •;■-  ■    •    , 


I  "••"■.  ^t  V 
li  .■••  ■*.*■•,  I'. 

■■••■":»:■■,:  ^ 

*U-'.5:...i: 

■■''  *■•■•■-, 
■  •■>  ■  ■  •     *  ••' 


represent;  and  for  that  reason  it  is  not  jn'ojiosed  to  draw 
any  inferences  in  favor  of  the  ori«j^inal  (U^sign  for  two  hun- 
dred feet  span  of  wliicli  this  truss  is  a  model ;  but  viewing 
these  results  in  tlie  liglit  of  a  eonipnrison  of  systems,  tlie 
evidence  is  undoubtedly  in  iiivor  of  the  triangular  web. 
This  after  all  is  but  the  veritication  by  experiment  of  the 
general  conclusions  arrived  at  in  tlie  earlier  discussion  of 
the  subject,  and  the  later  numerical  results  obtained  by  a 
comparison  of  strains. 

"We  therefore  feel  qualitied  in  passing  to  general  conclu- 
sions eminently  favorable  to  the  Isometrical  Truss.  And  in 
submitting  these  views  to  members  o/  the  profession  who 
may  }ionor  them  with  a  careful  inspection,  we  desire  again 
to  repeat  that  if  they  cs'Ji  be  demonstrated  to  l>e  clearly  j?t 
variance  with  correct  reasiuiing,  we  shall  be  only  too  glad 
to  protit  by  the  criticism. 

GENERAL     CONCLUSIONS. 

The  Isometrical  Truss,  founded  as  it  is  upon  the  most 
practically  accurate  web  system,  is  necessarily  the  most 
economical  form  of  parallel  girder  now  in  use,  either  of 
wood  or  iron.  And  from  a  coiiii>arison  of  four  well-known 
iron  bridges  as  instituted  by  V.  Shaler  Smith,  and  sub- 
mitted in  Table  III,  this  remark  nniv  be  extended  to  cover 
the  best  examples  of  suspension  trusses. 

Next  to  economy  of  construction  comes  the  question  of 
general  availability  for  all  spans  and  positions.  We  have 
not  thought  it  nee  .-sary  to  discuss  at  any  great  length  the 
comparative  me'it:'  of  the  different  trusses  in  this  respect, 
since  it  involves  a-.i  amount  of  detail  which  would  be  any- 
thing but  interesting,  and  at  best  quite  unnecessa'y  to  the 
formation  of  a  generally  accurate  oi»inion;  and  after  all  the 
J  great  desideratum  of  a  perfect  web  carries  with  it  advan- 
tages which  are  etjually  applicable  to  short  or  long  spans 
through  or  deck  bridires. 

COMPENSATION     AND    AD.JUSTMENT. 

As  to  the  elements  of  compensation  and  adjustment  in 
the  Isometrical  Truss,  we  have  seen  that  owing  to  the  ecjui- 


Bj.;'  tj 


THE    I8OMETRI0AL    TRUS8. 


83 


lateral  arrangements  of  its  internal  members,  no  possible 
distortion  of  figure  can  take  place  under  a  change  of  tem- 
perature; hence  the  relative  strength  of  the  structure 
always  remains  the  same.  In  the  iron  truss  the  only  effect 
of  a  high  temperature  is  to  increase  the  total  length  of  the 
chords;  but  in  the  application  of  the  principle  to  wood,  since 
one  of  the  diagonals  in  eacli  panel  must  be  iron  and  the 
other  wood,  the  effect  of  an  expansion  of  the  iron  is  simply 
to  lighten  the  original  camber  of  the  truss  to  a  sufficient 
extent  to  bring  the  brace  again  into  play,  and  the  rigidity 
of  the  structure  remains  unchanged.  In  fact  this  is  the 
only  case  in  which  compensation  is  rendered  necessary, 
since  in  the  iron  truss,  supposing  wrought  iron  to  be  used 
throughout,  all  parts  expand  equally,  and  consequently  no  . 
c'aml)or  is  lost. 

Adjustments  are  rendered  necessary  when,  by  reason  of 
the  shrinkage  of  a  portion  of  the  material,  as  in  a  wooden 
bridge,  or  from  defects  in  workmanship,  the  structure  is 
deflected  below  the  horizontal ;  and  to  effect  this  the  diag- 
onals pointing  downward  towards  the  centre  must  be  short- 
ened, since  we  have  seen  that  a  lengthening  of  these  mem- 
bers by  a  rise  in  temperature  will  cause  depression  in  a 
woodon  structure,  hence  reversing  the  operation  will  cer- 
tainly restore  the  proper  elevation.  It  would  scarcely  seem 
necessary,  after  the  general  discussion  of  the  action  of 
strains  in  the  web,  to  devote  more  time  to  the  question  of 
adjustment,  but  to  add  the  evidence  of  experiment  we  will 
mention  that  the  model  Isometrical  Truss  was  subjected  to 
a  variety  of  trials,  by  the  writer,  with  a  view  to  determine 
the  facility  with  which  it  might  be  restored  to  its  proper 
camber,  after  a  supposed  shrinkage.  All  the  diagonals 
were  brought  to  a  proper  bearing,  and  the  camber  of  the 
bottom  chord  noted  carefully,  afterwards  the  entire  truss >|j 
was  caused  to  assume  a  heavier  camber  by  simply  screw- 
ing up  the  diagomil  rods,  heretofore  mentioned,  with  the 
thumb  and  finger,  and  it  was  easily  seen  that  any  desired 
elevation  within  practical  limits  could  be  secured  by  this 
means.  On  the  other  hand,  the  application  of  a  wrench, 
with  great  force,  to  the  rods  of  the  model  Howe,  alongside. 


'mi 


W' 


1« 


:;*! 


->    • 


L  «i 


■■*■;  • 


.  i~.r. 


^ ,'.«, 


34 


THE    ISOMETRICAL   TRUSS. 


.      «?' 


had  no  other  eftect  than  to  sink  the  bolt  heads  and  their 
washers  into  the  chords,  and  no  change  of  camber  was  pro- 
duced Avhatever. 

The  large  plate  accompanying  this  paper  represents  one 
span  of  a  single-track  through  bridge,  now  being  built  by 
the  writer,  for  the  Perkiomen  Railroad  Company,  over  the 
Schuylkill  River  near  PhaMiixville.  All  the  parts  are  pro- 
portioned to  resist  the  strains,  as  represented  in  Fig.  8,  and 
particularly  in  the  braces,  is  a  large  margin  added  above 
the  requirements  for  safety.  It  will  also  be  noticed  that 
the  connections  over  the  pier  and  the  anchorage  involve  a 
considerable  expenditure  of  materia]  beyond  what  is  gen- 
erally called  for  in  the  best  railroad  bridges.  This  has  been 
added  as  a  measure  of  safety,  and  while  in  all  ordinary 
cases  this  item  of  expense  might  be  dispensed  with,  the 
increased  feeling  of  safety  in  the  reduction  of  the  effective 
span  twenty  feet,  and  the  perfect  immunity  from  the  action 
of  wind,  will  fully  compensate  for  the  increased  expendi- 
ture of  material. 

The  several  isometrical  sketches  will  serve  to  illustrate 
the  arrangement  of  braces  and  ties,  counter  braces  and 
counter  ties.  The  angle  block  is  believed  to  combine  ad- 
vantages over  either  the  original  oak  block  or  the  later 
cast-iron  Howe,  in  insuring  a  uniform  bearing  over  the 
entire  chord  without  the  danger  of  fracture,  as  in  cast-iron, 
or  of  rapid  decay,  as  in  the  simple  oak  bearing.  We  have 
proposed  locust  as  the  best  material  for  the  filling  of  this 
block,  but  tliere  are  many  other  species  of  wood  which 
would  answer  almost  as  well ;  circumstances  of  conve- 
nience had  much  to  do  with  its  selection  in  this  case. 

The  connection  of  the  diagonal  rods  with  the  bottom 
chord  by  means  of  a  2"  pin  directly  through  the  centre  of 
the  chord,  admits  of  a  considerable  saving  of  material  hi 
the  bearings,  and  the  application  is  only  rendered  possible 
by  the  reduced  strain  involved  in  this  system.  It  would,  of 
course,  be  out  of  the  question  to  admit  of  attachments  of 
this  kind  in  either  the  Howe  or  Whipple,  unless  the  chords 
were  considerably  increased  beyond  the  requirements  for 
horizontal  strain.     A  similar  remark  migh*:  be  extended 


^■HHIIHH 


THE    I80METR1CAL   TRUH8. 


35 


to  the  bearing  block  connections  on  the  top  and  bottom 
chords.  In  the  original  Howe  bearing  block,  it  was  found 
that  the  material  of  the  chord  was  eventually  crushed  and 
destroyed  at  the  plane  of  contact,  owing,  of  course,  to  the 
enormous  strain  on  the  braces  and  rods  near  the  points  of 
support ;  but  as  we  have  seen  that  the  Isometrical  princi- 
ple permits  a  reduction  of  50  per  cent,  of  this  strain,  it  will 
readily  be  admitted  that  this  element  of  safety  fully  com- 
pensates for  all  such  disturbing  causes.  In  packing  the 
chords  with  oak  blocks  and  keys,  the  writer  is  well  a  vare 
that  there  is  great  liability  to  rot  at  the  planes  of  junction, 
by  reason  of  the  deleterious  acids  contained  in  the  natural 
oak,  but  being  in  a  position  to  test  the  ettcct  of  the  Bur- 
uottizing  process,  In  neutralizing  this  destructive  tendency, 
the  experiment  is  made,  with  an  eye  to  its  future  applica- 
tioii  to  such  parts  of  the  truss  as  may  not  be  subjected  to 
transverse  strain.  But  inasmuch  as  the  annexed  plan  of 
bridge  was  intended  merely  as  an  example  of  tlie  applica- 
tion of  the  principles  involved  in  the  Isometrical  system, 
it  will  be  unnecessary  to  examine  further  into  practical 
details,  which, after  all,  must  be  left  to  the  test  of  extended 
experience,  and  which  cannot  affect  the  general  conclu- 
pions  already  siibmitted. 


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